Existing plain old telephone service (POTS), based on twisted pair telephone cable, is the most widespread communication infrastructure in the world. Technologies have been developed which permit the use of existing telephone cables for the high rate data transmission characteristics of digital communication. These include the digital multitone signal technology that allows the twisted pair telephone subscriber lines to be used for multi media and high-speed data communication. Asymmetrical digital subscriber line (ADSL) allows the transmission of data with a rate exceeding 8 Mb/s to a subscriber premises, and at a rate as high as 1 Mb/s in bi-directional communication. Such rates expand existing access capacity by 50 fold or more without the need for new cabling. ADSL can transform the existing public telephone network from one limited to voice, text and lower resolution graphics to a powerful, ubiquitous system capable of bringing multimedia, including full motion video, to every home.
An ADSL circuit includes an ADSL modem on each end of a twisted pair telephone line, creating three information channels—a high speed downstream channel, a medium speed duplex channel and a POTS channel. The POTS channel is separated from digital modems by filters, thus guaranteeing uninterrupted POTS, even if ADSL fails. The high speed downstream channels support a bit rate from about 1.5 to about 8 Mb/s, while duplex channels support rates which range between 16 to 1040 Kb/s. Downstream data rates depend on a number of factors including the lengths of the copper line, the wire gauge, the presence of bridged taps and cross cable interference. Line attenuation obviously increases with line length and frequency and decreases as diameter increases. A typical ADSL line will transmit at the rate of 1.5 Mb/s, with a wire diameter of 0.5 mm, over 5.5 km and at 8 Mb/s over a distance of 3.7 km for a wire of the same diameter. For wire with a 0.4 mm diameter, the respective distances are 4.6 km and 2.7 km.
One problem of ADSL systems is the need to rewire existing telephone home networks within a subscriber premises and to place special splitter devices for separating voice and ADSL signals to a subscriber premises. In order to eliminate splitter and rewiring of home networks, a G.Lite ADSL system was developed, in which the separation between the ADSL and the voice signals is realized by means of special micro filters placed serially in the line connecting each home telephone device to the external line. However, the G.Lite system supports a bit rate of up to 1.5 Mb/s only in a downstream direction, which is too slow for a variety of applications including, in particular, video-on-demand service. Furthermore, the micro filter associated with a telephone device decreases the quality of voice communication.
In multi-apartment buildings, the telephone lines typically reach a central box and from there telephone lines extend to each of the apartments. In existing systems, such inter-building wiring is also not suitable for high-speed data communication. Typically, such inter-building wiring makes use of flat pair cables, which have unpredictable characteristics and are highly sensitive to RF noise.
Another problem of existing ADSL systems is that a customer must have an ADSL home modem and a personal computer at the subscriber premises. Every home device which requires high-speed data service from a telephone station, such as a video phone, digital TV, hi-fi digital audio, etc., must be connected to an ADSL home modem through a computer, typically a personal computer (PC). In practice, this means that a location, to make use of an ADSL system with intra-location network capability, needs two independent networks: an existing telephone network and an additional digital data network.
A further problem of ADSL systems is that only one home modem may communicate with a modem at a central office of the communication service provider at the same time. If a subscriber has several computers, only one of them may thus be connected to the telephone line.